1. Field of the Invention
The present invention relates to a fixing apparatus for use in an image forming apparatus and an image forming apparatus using the fixing apparatus, and in particular relates to a structure for reinforcing a heating roller of a fixing apparatus.
2. Discussion of the Background
In image forming apparatuses, such as copying machines, facsimile apparatus, printers, printing apparatus, etc., an unfixed toner image transferred onto and carried on a recording medium such as paper is heated by a fixing apparatus to be fixed onto the recording medium, and thereby a reproduced or printed image is obtained on the recording medium.
A background fixing apparatus has a configuration in which a pair of rollers are arranged to oppose each other. One of the rollers is used as a heating roller for heating a recording medium and the other roller is used as a pressing roller for pressing the recording medium against the heating roller. In this configuration, the recording medium is conveyed while being sandwiched between the heating roller and the pressing roller at a nip portion between the heating roller and the pressing roller, and thereby an unfixed toner image carried on the recording medium is fixed onto the recording medium by heat from the heating roller.
In order to accomplish efficient fixing in such a fixing apparatus, it is important to increase heat transmission efficiency of the heating roller by using material having a satisfactory heat conductivity for the heating roller. In addition, the heating roller is generally designed in such a way that the heat conductivity is increased. As an example, a wall thickness of the heating roller is made thin so that the heat capacitance of the heating roller is reduced, thereby enabling shortening the rising time for the heating roller to reach a fixing temperature and reducing the power consumption of the heating roller.
However, when a heating roller is constructed to have a thin wall, the mechanical rigidity of the heating roller is decreased, so that bending may occur in the heating roller or a portion of the heating roller pressed by a pressing roller may be crushed in an axial direction of the heating roller.
In a background construction for avoiding the above-described problem in a heating roller, separate reinforcing members such as rings are arranged to be fixed to internal surfaces of the heating roller at a plurality of positions of the heating roller, e.g., at a center part and end parts of the heating roller in an axial direction thereof, where the heating roller most likely tends to be crushed.
However, the present inventor recognized that in the above-described configuration, the reinforcing rings must be processed separately from the heating roller, thereby causing the cost of the heating roller to increase. In addition, when the heating roller is actually used in an image forming apparatus, the rings may be deformed due to thermal expansion thereof, thereby falling off the internal surfaces of the heating roller. As a result, deformation such as bending or crushing of the heating roller cannot be avoided. If the heating roller is deformed, the force of sandwiching a recording medium with the fixing roller and the pressing roller is decreased, and thereby the efficiency of heating the recording sheet with the heating roller is decreased, and consequently inferior fixing is caused.
Further, the present inventor recognized that when the reinforcing rings are arranged at a plurality of positions of a heating roller in an axial direction thereof, depending upon the number of the rings that are provided, heat loss may be caused by heat dissipation via the reinforcing rings, which may also cause a decrease in the efficiency of heating of the recording medium with the heating roller and consequent inferior fixing.
The present invention has been made in view of the above-discussed and other problems and addresses the above-discussed and other problems.
Preferred embodiments of the present invention provide a novel fixing apparatus and an image forming apparatus using the fixing apparatus, in which increased processing cost of a heating roller and falling off of reinforcing members for the heating roller are prevented and at the same time the rising time of the fixing apparatus to a fixing temperature can be decreased and inferior fixing is prevented.
According to a preferred embodiment of the present invention, a novel fixing apparatus for fixing a toner image carried on a recording sheet onto the recording sheet with heat includes a fixing roller having a plurality of ribs formed at a sheet passing area of the fixing roller in an axial direction of the fixing roller to protrude from an internal circumferential surface of the fixing roller toward a cross-sectional center thereof. The plurality of ribs are provided along the axial direction of the fixing roller such that a number of ribs per unit length at a center part of the sheet passing area of the fixing roller in the axial direction of the fixing roller is different from that at other parts of the sheet passing area. The fixing apparatus further includes a heating source for heating the fixing roller and a pressing roller opposing and rotating with the fixing roller, and the toner image on the recording sheet is fixed onto the recording sheet while the recording sheet is being sandwiched and conveyed between the fixing roller and the pressing roller.
In the above-described fixing apparatus, the heating source may be provided inside the fixing roller.
Further, the number of ribs per unit length may be greater at the center part of the sheet passing area of the fixing roller in the axial direction of the fixing roller than at the other parts of the sheet passing area.
Furthermore, a center of the fixing roller in the axial direction thereof may correspond to a center of the sheet passing area of the fixing roller in the axial direction of the fixing roller.
Still further, the fixing roller may have a first wall thickness at the sheet passing area thereof and a second wall thickness at least at a part of axial end parts thereof that are out of the sheet passing area. The second wall thickness may be greater than the first wall thickness.
Furthermore, the above-described fixing apparatus may further include a supporting member opposing a part of the axial end parts of the fixing roller to support the fixing roller. In this case, the part of the axial end parts of the fixing roller where the fixing roller supporting member opposes does not have the second wall thickness. Further, a rib may be formed at the part of each of the axial end parts of the fixing roller where one of the fixing roller supporting members is located to protrude from the internal circumferential surface of the fixing roller toward the cross-sectional center thereof. A height from the internal circumferential surface of the fixing roller, of the rib formed at the part of each of the axial end parts of the fixing roller where one of the fixing roller supporting members is located to protrude from the internal circumferential surface of the fixing roller toward the cross-sectional center thereof is different from that of the part of the axial end parts of the fixing roller having the second wall thickness. The height from the internal circumferential surface of the fixing roller, of the rib formed at the part of the axial end parts of the fixing roller where the fixing roller supporting member is located may be equal to that of the plurality of ribs formed at the sheet passing area of the fixing roller.
According to another preferred embodiment of the present invention, an image forming apparatus includes an image forming unit for forming a toner image on a recording sheet and a fixing apparatus for fixing the toner image on the recording sheet onto the recording sheet with heat. The fixing apparatus includes a fixing roller having a plurality of ribs formed at a sheet passing area of the fixing roller in an axial direction of the fixing roller so as to protrude from an internal circumferential surface of the fixing roller toward a cross-sectional center thereof. The plurality of ribs are provided along the axial direction of the fixing roller such that a number of ribs per a unit length at a center part of the sheet passing area of the fixing roller in the axial direction of the fixing roller is different from that at other parts of the sheet passing area. The fixing apparatus further includes a heating source for heating the fixing roller and a pressing roller opposing and rotating with the fixing roller, and the toner image on the recording sheet is fixed onto the recording sheet while the recording sheet is being sandwiched and conveyed between the fixing roller and the pressing roller.
In the above-described image forming apparatus, the heating source may be provided inside of the fixing roller.
Further, the number of ribs per unit length may be greater at the center part of the sheet passing area of the fixing roller in the axial direction of the fixing roller than at the other parts of the sheet passing area.
Furthermore, a center of the fixing roller in the axial direction thereof may correspond to a center of the sheet passing area of the fixing roller in the axial direction of the fixing roller.
Still furthermore, the fixing roller may have a first wall thickness at the sheet passing area thereof and a second wall thickness at least at a part of axial end parts thereof that are out of the sheet passing area. The second wall thickness is greater than the first wall thickness.
Further, in the above-described image forming apparatus, the fixing apparatus may include a supporting member opposing a part of the fixing roller at the axial end parts of the fixing roller to support the fixing roller. In this case, the part of each of the axial end parts of the fixing roller where the fixing roller supporting member is located does not have the second wall thickness.
Further, a rib may be formed at the part of each of the axial end parts of the fixing roller where one of the fixing roller supporting members is located to protrude from the internal circumferential surface of the fixing roller toward the cross-sectional center thereof. In this case, a height from the internal circumferential surface of the fixing roller, of the rib formed at the part of each of the axial end parts of the fixing roller where the fixing roller supporting member is located to protrude from the internal circumferential surface of the fixing roller toward the cross-sectional center thereof is different from that of the parts of the axial end parts of the fixing roller, having the second wall thickness. The height from the internal circumferential surface of the fixing roller of the rib formed at the part of each of the axial end parts of the fixing roller where one of the fixing roller supporting members is located may be equal to that of the plurality of ribs formed at the sheet passing area of the fixing roller.